Circular
154Guidebook to coal geology of northwest New Mexico

Contains geologic road logs for a three-day field
trip on the coal resources of northwestern New Mexico. Also contains three
short technical papers pertaining to subject. The San Juan Basin is a structural
depression at the edge of the Colorado Plateau. The basin has about 6,000
ft of structural relief and covers about 10,000 mi2 of northwestern New
Mexico and southeastern Colorado. Monoclines are the most distinct structures
in the Colorado Plateau; excellent examples occur in the San Juan Basin,
the most prominent being the north-south-trending Hogback Mountain, near
the west edge of the basin. Over 14,000 ft of sedimentary rocks occur in
the deepest part of the basin, as indicated in a well drilled to basement
near Gobernador in northwestern Rio Arriba County.

Jurassic and older rocks mark the perimeter of the basin. Within the basin,
broad bands of Cretaceous and younger rock units crop out. The Cretaceous-Tertiary
boundary occurs within the Ojo Alamo Sandstone. The central and eastern
areas are covered by rocks of Tertiary age. Quaternary deposits occur in
stream valleys, in terraces, along mountain fronts, and atop all other rock
units. The general stratigraphic succession in the San Juan Basin is shown
in the chart presented inside the back cover. The uranium-bearing Jurassic
rocks are believed to record nonmarine or continental deposition extending
northward from highlands in the south. The Cretaceous portion of the sedimentary
record of the basin is summarized in the cross section. The coal-bearing
strata are believed to represent deposition in, and at the margins of, the
vast Cretaceous seaway that extended across the continent from the Arctic
Ocean to the Gulf of Mexico about 135 million years ago. As this sea lapped
on and off the land, a unique record of alternating carbonaceous shales
and coals of coastal plain origin, marine shoreline sandstones, and marine
offshore shales was produced. Nonmarine conditions were restored after the
sea retreated at the end of Cretaceous time resulting in the Tertiary continental
deposits in the central basin.

In physiographic terms, the San Juan Basin is situated in the Navajo section
of the Colorado Plateau and is characterized by broad open valleys, mesas,
buttes, and hogbacks. Away from major valleys and canyons topographic relief
is generally low. Native vegetation is sparse and shrubby. Drainage is mainly
by the San Juan River, the only permanent stream in the Navajo Section of
the Colorado Plateau; the San Juan River is a tributary of the Colorado
River. Between the inflow point in Rio Arriba County and outflow point in
San Juan County, the San Juan drops about 1,800 ft in elevation. Major tributaries
include the Animas, Chaco, and La Plata Rivers. Average annual discharges
reported for the period 19131951 are: 729,400 acre-ft for the Animas
at Farmington; 1,179,400 acre-ft for the San Juan near Blanco; and 2,194,800
acre-ft for the San Juan at Shiprock. Flow of the San Juan River across
the basin is regulated by the Navajo Dam, located about 30 mi northeast
of Farmington. The climate is arid to semiarid with an average annual precipitation
of 810 in. The annual pan evaporation rate, based on records for the
period 19481962, is 67 in. The average January temperature is 28ºF;
the average July temperature is 73ºF.

Coal occurs throughout the Cretaceous section of the basin. In New Mexico
important deposits are largely restricted to the Mesaverde Group and the
Fruitland Formation. The Mesaverde Group includes, in ascending order, the
Gallup Sandstone, the Crevasse Canyon Formation, the Point Lookout Sandstone,
the Menefee Formation, and the Cliff House Sandstone. Only the Crevasse
Canyon and Menefee Formations are significantly coal bearing. Minor coal
occurs in the Gallup and Dakota Sandstones. Strippable reserves approach
6 billion tons. The largest portion of this coal is associated with Fruitland
Formation. Nearly equal reserves occur in the overburden categories of 10150
ft and 150250 ft. Reserves of deep coal, defined as that beneath more
than 250 ft of overburden, are even larger. The Fruitland alone is estimated
to contain about 154 billion tons of deep coal-about 30 times the strippable
reserves. The Menefee Formation contains another 115 billion tons of deep
coal.